Method and desulfurizing nitrogen oxide storage catalysts in the exhaust gas system of a lean mix engine

ABSTRACT

Lean burn engines require an exhaust gas system comprising nitrogen oxide storage catalysts for removal of nitrogen oxides from their exhaust gases. When the lean burn engine is operated with a sulphur-containing exhaust gas, the storage catalysts have to be desulphurized from time to time. During the desulphurization, there is the risk of high pollutant emissions. These emissions can be reduced when the cylinders of the lean burn engine are combined in two groups which release their exhaust gas to two assigned exhaust legs, in each of which is arranged at least one nitrogen oxide storage catalyst. The two exhaust legs are combined beyond the storage catalysts to form a common exhaust leg which contains a catalyst which possesses a three-way function under stoichiometric conditions. The two nitrogen oxide storage catalysts are desulphurized offset in time with respect to one another. While rich exhaust gas at a high temperature flows through one storage catalyst for desulphurization, lean exhaust gas flows through the second storage catalyst, in such a way that the combined exhaust gas is of stoichiometric composition over the entire desulphurization period. Under the stoichiometric conditions, the catalyst with three-way function is capable of converting hydrocarbons, carbon monoxide and nitrogen oxides to harmless components.

The invention relates to a method for desulphurizing nitrogen oxide storage catalysts in the exhaust gas system of a lean burn engine with two or more cylinders.

Lean burn engines refer to diesel engines, and also to petrol engines with direct petrol injection and CNG engines (compressed natural gas=methane) which can be operated under lean conditions. To remove nitrogen oxides from the exhaust gas of lean burn engines, what are known as nitrogen oxygen storage catalysts can be used.

During its storage phase, a nitrogen oxide storage catalyst oxidizes the nitrogen monoxide present in the lean exhaust gas to nitrogen dioxide and then stores it in the form of nitrates. The mode of operation of nitrogen oxide storage catalysts is described in detail in the SAE publication SAE 950809. For oxidation of nitrogen monoxide, a storage catalyst contains, as catalytically active components, usually platinum with or without palladium. For storage of the nitrogen oxides as nitrates, basic oxides, carbonates or hydroxides of alkali metals, alkaline earth metals and rare earth metals are used; preference is given to using basic compounds of barium and of strontium.

After exhaustion of its storage capacity, a storage catalyst has to be regenerated during a regeneration phase. To this end, the exhaust gas is briefly enriched, for example by operating the engine with a rich air/fuel mixture. In the rich exhaust gas, the nitrogen oxides are desorbed again and reduced to nitrogen over the catalytically active components with the aid of the rich exhaust gas constituents. For this purpose, the storage catalyst usually contains rhodium in addition to the platinum.

Storage phase and regeneration phase alternate regularly. The alternation of storage phase and regeneration phase is referred to as alternating rich/lean operation. The storage phase usually lasts between 60 and 200 seconds, whereas the duration of the regeneration phase is only between 1 and 10% of the storage phase and thus comprises only a few seconds.

The function of nitrogen oxide storage catalysts is impaired by sulphur compounds which are present in the fuel and motor oil and get into the exhaust gas essentially in the form of sulphur dioxide in the course of combustion, and are bound by the nitrogen oxide storage catalysts in the form of very stable sulphates. This is at the expense of the nitrogen oxide storage capacity. At high sulphur contents in the fuel (>10 ppm), nitrogen oxide storage catalysts therefore frequently have to be desulphurized. To this end, the exhaust gas has to be brought to desulphurizing conditions, i.e. it has to be enriched and its temperature has to be raised. The air/fuel ratio lambda λ of the exhaust gas should be lowered to a value below 0.98, preferably to below 0.95, and the exhaust gas temperature should be brought to a value between 600 and 750° C. Under these conditions, the sulphates formed are decomposed and emitted as hydrogen sulphide or preferably as sulphur dioxide.

When a nitrogen oxide storage catalyst is contacted with a sulphur-containing exhaust gas, the storage catalyst thus, as well as the regular regeneration to remove the nitrogen oxides stored, also has to be desulphurized from time to time in order to reverse a continuous deterioration in the nitrogen oxide storage capacity as a result of sulphates formed. The interval between two desulphurizations depends on the sulphur content of the fuel, but even at high sulphur contents is generally still several operating hours of the engine and is thus significantly greater than the interval between two regenerations to remove the nitrogen oxides stored. For the desulphurization, usually 2 to 10 minutes are required. It thus likewise lasts longer than the nitrogen oxide regeneration of the storage catalyst.

The frequent desulphurization is at the expense of fuel consumption and leads, owing to the necessary high exhaust gas temperatures, to rapid ageing of the catalysts. Therefore, motor vehicles with lean burn petrol engines have to date been sold only on the European market, since fuels with a sulphur content of less than 10 ppm are supplied here. In the USA, the emissions legislation is particularly strict, but the sulphur content in the fuel for petrol engines here is at present still up to 30 ppm. In other regions, the sulphur content in the fuel is still significantly higher.

The development of motor vehicles with lean burn petrol engines for markets with a high sulphur content in the fuel thus has to take into account that, in this case, the nitrogen oxide storage catalysts have to be desulphurized frequently. In addition to the disadvantages of frequent desulphurization which have already been mentioned, namely the increased fuel consumption and the high thermal stress on the catalysts, a further disadvantage which occurs is a high emission of hydrocarbons and nitrogen oxides during the desulphurization, since the rich exhaust gas during the desulphurization contains high concentrations of uncombusted hydrocarbons, carbon monoxide and nitrogen oxides, and also ammonia formed from the nitrogen oxides over the catalysts, but barely any oxygen to convert these exhaust gas components over the catalysts. They are therefore released to the environment in uncleaned form as pollutants.

However, American emissions legislation stipulates that the limits for hydrocarbons, carbon monoxide and nitrogen oxides, which were already very low in any case, have to be complied with even taking account of the desulphurization of the nitrogen oxide storage catalysts. For this purpose, the emissions during the desulphurization of nitrogen oxide storage catalysts are applied to the entire driving cycle envisaged for the emissions measurements. It has been found that even the emissions during a single desulphurization of nitrogen oxide storage catalysts can exceed the stipulated limits for so-called SULEVs (SULEV=Super Ultra Low Emission Vehicle).

It is an object of the present invention to specify a method for desulphurizing nitrogen oxide storage catalysts which largely suppresses the increased pollutant emissions during the desulphurization and thus makes it possible to comply with limits for lean burn internal combustion engines even in the case of sulphur-containing fuels.

This object is achieved by the method described by Claim 1. The method requires a lean burn engine with two or more cylinders, which are divided into a first group and a second group. The exhaust gases of the two cylinder groups are released into exhaust legs assigned to each. Each exhaust leg contains at least one nitrogen oxide storage catalyst for removal of the nitrogen oxides in the exhaust gas. The two exhaust legs open downstream of the storage catalysts into a common exhaust leg at a confluence. For further aftertreatment of the exhaust gas, the common exhaust leg contains a catalyst which, under stoichiometric conditions, has a three-way function, which allows it to simultaneously remove hydrocarbons, ammonia, nitrogen oxides and carbon monoxide from the stoichiometric exhaust gas. This catalyst is referred to hereinafter as three-way catalyst for short.

The lean burn engine can be configured as an in-line engine in which all cylinders are arranged in succession in a single cylinder bank. Alternatively, each group of cylinders can be combined in a separate cylinder bank.

According to the invention, the nitrogen oxide storage catalysts in the two exhaust legs are desulphurized offset in time with respect to one another. The desulphurization conditions needed for this purpose can be established by engine measures or by external measures. The engine measures include the operation of the group of cylinders assigned in each case with a rich air/fuel mixture, the postinjection of fuel, a late combustion position or a multistage combustion. These measures can also be combined with one another. For external establishment of the desulphurization conditions, the exhaust gas can be enriched by injecting fuel into the particular exhaust leg upstream of the nitrogen oxide storage catalyst, and its temperature can be raised to desulphurization temperature, for example, by external heating. The external heating can also be undertaken by means of oxidation catalysts arranged upstream of the nitrogen oxide storage catalysts and combustion of the fuel injected on these catalysts.

During the desulphurization of one nitrogen oxide storage catalyst, the other nitrogen oxide storage catalyst is operated under lean exhaust gas conditions of the lean burn engine. The air/fuel ratios of the exhaust gases in the two exhaust legs are adjusted with respect to one another such that the exhaust gas in the common exhaust leg ideally has an air/fuel ratio lambda of 1 over the entire desulphurization time, i.e. is of stoichiometric composition. In the real case, the air/fuel ratio in the common exhaust leg will deviate downward or upward from the ideal value, to a greater or lesser degree, variably with time, owing to the dynamic operating conditions of the engines.

The other nitrogen oxide storage catalyst is desulphurized in a corresponding manner offset in time with respect to the first nitrogen oxide storage catalyst. Between two desulphurizations, the two nitrogen oxide storage catalysts are operated in the known alternating operation between storage phase and regeneration phase.

For the desulphurization of one nitrogen oxide storage catalyst, the exhaust gas is enriched to an air/fuel ratio lambda of <1, preferably to <0.98 and particularly to <0.95. At the same time, the second storage catalyst is operated at an air/fuel ratio of the exhaust gas lambda of >1, preferably >1.1. After the combination of the two exhaust gas streams, the combined exhaust gas has an air/fuel ratio of about lambda=1. Under these conditions, the three-way catalyst in the common exhaust leg can virtually completely eliminate the pollutant components from the rich exhaust gas of one exhaust leg with the pollutant components from the lean exhaust gas of the other exhaust leg.

A further advantage of the invention is that the desulphurization can be performed under constantly rich exhaust gas conditions. This is because the hydrogen sulphide formed is converted to sulphur dioxide over the three-way catalyst in the common exhaust leg. In contrast, in the desulphurization methods known from the prior art, the exhaust gas is usually switched back and forth between lean and rich in rapid alternation, in order to suppress the formation of hydrogen sulphide. However, this alternating operation requires high exhaust gas temperatures for the desulphurization and leads to higher fuel consumption and longer desulphurization times compared to the method described here.

During the desulphurization of one storage catalyst, the second storage catalyst can be operated with constantly lean exhaust gas. The regular nitrogen oxide regeneration of the second storage catalyst during the desulphurization of the first storage catalyst is unnecessary. Although the storage capacity of the second storage catalyst for nitrogen oxides is already exhausted about 1 to 2 minutes after the start of desulphurization of the first storage catalyst, the nitrogen oxides which therefore break through the nitrogen oxide storage catalyst are completely converted by the downstream three-way catalyst.

The catalyst in the common exhaust leg must be able to fulfil the function of a three-way catalyst under stoichiometric exhaust gas conditions. For this purpose, it contains at least one noble metal from the group of platinum, palladium and rhodium. The catalyst preferably contains palladium and/or rhodium. In addition, the catalyst may contain so-called oxygen storage materials, particularly cerium oxide or a mixed oxide containing cerium oxide. Preference is given to using a catalyst configured specially as a three-way catalyst. Alternatively, however, instead of a three-way catalyst, a nitrogen oxide storage catalyst can be used in the common exhaust leg. This fulfils the same purpose as a three-way catalyst when the exhaust gas is of stoichiometric composition, i.e. possesses an air/fuel ratio of lambda=1. In the normal operation of the emission control system, the storage catalyst may contribute additionally to the conversion of nitrogen oxides by storing them during the lean phase and converting them to nitrogen by means of short rich pulses.

The stipulation of the air/fuel ratio lambda=1 for the exhaust gas in the common exhaust leg during the desulphurization should of course be understood only within the context of the tolerances customary for this purpose of ±0.04, preferably ±0.02. In particular cases, it may even be advantageous to set the air/fuel ratio in the common exhaust leg at a slightly rich or slightly lean level within the tolerance range specified. A slightly lean exhaust gas in the common exhaust leg may be advantageous when the hydrogen sulphide formed in the desulphurization is to be oxidized to sulphur dioxide over the three-way catalyst. When a nitrogen oxide storage catalyst is used instead of a true three-way catalyst, a slightly rich exhaust gas can prevent the sulphur dioxide or hydrogen sulphide formed in the desulphurization from being absorbed by the nitrogen oxide storage catalyst in the common exhaust leg to form sulphates.

For exact regulation of the required exhaust gas composition in the common exhaust leg, it is advisable to arrange an oxygen probe upstream and/or downstream of the three-way catalyst. This probe passes its lambda signal on to an engine control system. When the desulphurization conditions are established through engine measures, the combustion in the two cylinder groups is conducted such that a very substantially stoichiometric exhaust gas mixture is present in the common exhaust leg. Suitable oxygen probes are linear lambda probes or so-called jump probes. Nitrogen oxide probes can also be used to measure the oxygen content.

When engine measures to establish the desulphurization conditions are undesirable or impossible, as may be the case, for example, in diesel engines, rich or lean exhaust gas mixtures in the particular exhaust legs can also be established during the desulphurization by direct injection of fuel into the particular exhaust legs.

The invention is illustrated in detail with reference to FIGS. 1 and 2. The figures show:

FIG. 1: emission control system for performing the method for desulphurization with reduced emission of pollutants

FIG. 2: a further embodiment of the emission control system for performance of the method for desulphurization with reduced emission of pollutants

FIG. 3: schematic diagram of the offset operation of the two cylinder banks of the emission control systems according to FIGS. 1 and 2

FIG. 1 shows an emission control system for performing the desulphurization method with reduced pollutant emission. Reference numeral (1) denotes a lean burn engine with two cylinder banks (2) and (2′). The exhaust gases of these cylinder banks are released into the two exhaust legs (3) and (3′). At the confluence (4), the two exhaust gas lines (3) and (3′) are combined to form a common exhaust leg (5). For storage and conversion of the nitrogen oxides emitted by the lean burn engine (1), the nitrogen oxide storage catalysts (6) and (6′) are arranged in the exhaust legs (3) and (3′). The three-way catalyst or nitrogen oxide storage catalyst (7) is present in the common exhaust leg. Reference numerals (8) and (8′) denote the possible positions of an oxygen probe (lambda probe).

To desulphurize the nitrogen oxide storage catalyst (6), the cylinders of the cylinder bank (2) are operated with a rich air/fuel mixture by an engine control system which is not shown. This leads to an exhaust gas with an air/fuel ratio less than 1, whose temperature is raised to the necessary desulphurization temperature of about 700° C., for example by postinjection. Over the entire desulphurization period of about 2 to 10 minutes, the cylinders of the second cylinder bank (2′) are operated with a lean air/fuel mixture. The correspondingly lean exhaust gas with lambda greater than 1 has a temperature of 300 to 400° C. which is optimal for the nitrogen oxide storage catalyst. At the confluence of the two exhaust legs, the two exhaust gas streams are mixed and lead to a combined exhaust gas with a temperature between the desulphurization temperature and the normal exhaust gas temperature. The oxygen content of the combined exhaust gas is measured with the oxygen probes (8) and/or (8′) and regulated with the aid of the engine control system to give a value of the air/fuel ratio as close as possible to 1.

FIG. 2 shows a variant of the emission control system for performing the method. Upstream of the nitrogen oxide storage catalysts (6) and (6′), a further catalyst (9) and (9′) is inserted into each exhaust leg. This may be a further nitrogen oxide storage catalyst, a three-way catalyst or an oxidation catalyst. All three catalyst types can further reduce the pollutant emission of the emission control system. In the case of diesel engines, it may be advantageous to arrange a diesel particulate filter, with or without a catalytic coating, between the catalysts (9) and (6), and between (9′) and (6′), or beyond each of catalysts (6) and (6′).

FIG. 3 is a schematic diagram of the offset operation of the two cylinder banks (2) and (2′) of the emission control systems of FIGS. 1 and 2 as a function of the operating time t. The brief desulphurization of catalyst (6) is always undertaken during the normal rich/lean alternating operation of catalyst (6′), and vice versa. For this purpose, the mode of operation of the two cylinder banks (2) and (2′) is correspondingly switched as described above. 

1. Method for desulphurization nitrogen oxide storage catalysts in the exhaust gas system of a lean burn engine (1) with two or more cylinders, wherein the cylinders of the lean burn engine are divided into a first group (2) and a second group of cylinders (2′) which release their exhaust gases into first (3) and second exhaust legs (3′) assigned to each, at least one nitrogen oxide storage catalyst (6) and (6′) being arranged in each exhaust leg and the two exhaust legs being combined downstream of the storage catalysts at a confluence (4) to form a common exhaust leg (5) which contains a catalyst (7) which, under stoichiometric conditions, possesses a three-way function, the first nitrogen oxide storage catalyst (6) being desulphurized by enriching the exhaust gas in the first exhaust leg (3) and raising its temperature to desulphurization temperature, while the exhaust gas in the second exhaust leg (3′) is kept constantly lean, the exhaust gases in the two exhaust legs being adjusted with respect to one another such that the exhaust gas in the common exhaust leg (5) has an air/fuel ratio of about lambda=1 over the entire desulphurization period, and the second nitrogen oxide storage catalyst (6′) being desulphurized in a corresponding manner offset in time with respect to the first nitrogen oxide storage catalyst (6).
 2. Method according to claim 1, wherein, for desulphurization of the storage catalysts, the exhaust gas is enriched by engine measures and its temperature is brought to desulphurization temperature.
 3. Method according to claim 2, wherein the engine measures are selected from the operation of the group of cylinders assigned in each case with a rich air/fuel mixture, the postinjection of fuel, a late combustion position, a multistage combustion or a combination of these measures.
 4. Method according to claim 1, wherein, for desulphurization of the storage catalysts, the exhaust gas is enriched by injecting fuel into the particular exhaust leg upstream of the nitrogen oxide storage catalyst, and its temperature is raised to desulphurization temperature by external heating.
 5. Method according to claim 1, wherein, an oxygen probe is arranged upstream and/or downstream of the catalyst with three-way function for regulation of the air/fuel ratio in the common exhaust leg. 